U.S. patent application number 11/387977 was filed with the patent office on 2007-09-06 for additive building material mixtures containing microparticles swollen in the building material mixture.
This patent application is currently assigned to ROEHM GBMH & CO. KG. Invention is credited to Thorsten Goldacker, Holger Kautz, Gerd Lohden, Jan Hendrik Schattka.
Application Number | 20070208107 11/387977 |
Document ID | / |
Family ID | 37964600 |
Filed Date | 2007-09-06 |
United States Patent
Application |
20070208107 |
Kind Code |
A1 |
Schattka; Jan Hendrik ; et
al. |
September 6, 2007 |
Additive building material mixtures containing microparticles
swollen in the building material mixture
Abstract
The present invention relates to the use of polymeric
microparticles in hydraulically setting building material mixtures
for the purpose of enhancing their frost resistance and cyclical
freeze/thaw durability.
Inventors: |
Schattka; Jan Hendrik;
(Hanau, DE) ; Goldacker; Thorsten; (Rossdorf,
DE) ; Kautz; Holger; (Hanau, DE) ; Lohden;
Gerd; (Hanau, DE) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
ROEHM GBMH & CO. KG
Darmstadt
DE
|
Family ID: |
37964600 |
Appl. No.: |
11/387977 |
Filed: |
March 24, 2006 |
Current U.S.
Class: |
523/200 ;
524/2 |
Current CPC
Class: |
C04B 28/02 20130101;
C04B 28/02 20130101; C04B 20/008 20130101; C04B 24/2641 20130101;
C04B 2103/0065 20130101; C04B 24/2641 20130101; C04B 28/02
20130101; C04B 24/2664 20130101; C04B 2103/0049 20130101; C04B
2111/29 20130101; C04B 24/2641 20130101; C04B 24/2641 20130101;
C04B 16/085 20130101; C04B 2103/0057 20130101; C04B 20/1029
20130101; C04B 20/008 20130101; C04B 20/1029 20130101; C04B 16/085
20130101 |
Class at
Publication: |
523/200 ;
524/2 |
International
Class: |
C04B 24/26 20060101
C04B024/26; C08K 9/00 20060101 C08K009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 1, 2006 |
DE |
10 2006 009 842.0 |
Claims
1. Use of polymeric core/shell microparticles in hydraulically
setting building material mixtures, characterized in that they
possess a core that can be swollen by bases, and in that their
shell is composed of polymers having a glass transition temperature
of below 50.degree. C.
2. Use of polymeric core/shell microparticles in hydraulically
setting building material mixtures, according to claim 1,
characterized in that their shell is composed of polymers having a
glass transition temperature of below 30.degree. C.
3. Use of polymeric core/shell microparticles according to claim 1,
characterized in that the core is swollen before the particles are
added to the building material mixture.
4. Use of polymeric core/shell microparticles according to claim 1,
characterized in that the core is swollen `in situ` in the alkali
medium of the building material mixture.
5. Use of polymeric core/shell microparticles according to claim 1,
characterized in that the microparticles are composed of polymer
particles which comprise a polymer core (A), which is swollen or
swellable by means of an aqueous base and contains one or more
unsaturated carboxylic acid (derivative) monomers, and a polymer
envelope (B), which is composed predominantly of nonionic,
ethylenically unsaturated monomers.
6. Use of polymeric core/shell microparticles according to claim 5,
characterized in that the nonionic, ethylenically unsaturated
monomers of the shell are composed of styrene, butadiene,
vinyltoluene, ethylene, vinyl acetate, vinyl chloride, vinylidene
chloride, acrylonitrile, acrylamide, methacrylamide and/or C1-C12
alkyl esters of acrylic or methacrylic acid.
7. Use of polymeric core/shell microparticles according to claim 5,
characterized in that the unsaturated carboxylic acid (derivative)
monomers of the core (A) are selected from the group of acrylic
acid, methacrylic acid, maleic acid, maleic anhydride, fumaric
acid, itaconic acid and crotonic acid.
8. Use of polymeric core/shell microparticles according to claim 1,
characterized in that the microparticles have a polymer content of
2% to 98% by weight.
9. Use of polymeric core/shell microparticles according to claim 1,
characterized in that the shell (B) accounts for 10% to 96% by
weight of the total weight of the particle.
10. Use of polymeric core/shell microparticles according to claim
1, characterized in that the microparticles have an average
particle size of 100 to 5000 nm.
11. Use of polymeric core/shell microparticles according to claim
10, characterized in that the microparticles have an average
particle size of 200 to 2000 nm.
12. Use of polymeric core/shell microparticles according to claim
11, characterized in that the microparticles have an average
particle size of 250 to 1000 nm.
13. Use of polymeric core/shell microparticles according to claim
1, characterized in that the microparticles are used in an amount
of 0.01% to 5% by volume, based on the building material
mixture.
14. Use of polymeric core/shell microparticles according to claim
13, characterized in that the microparticles are used in an amount
of 0.1% to 0.5% by volume, based on the building material
mixture.
15. Use of polymeric core/shell microparticles according to claim
1, characterized in that the building material mixtures are
composed of a binder selected from the group of cement, lime,
gypsum and anhydrite.
16. Use of polymeric core/shell microparticles according to claim
1, characterized in that the building material mixtures are
concrete or mortar.
17. A process for preparing a building material mixture which after
hardening is resistant to frost and to cyclical freeze/thaw,
characterized in that swellable but unswollen core/shell particles
are mixed with the remaining components of the building material
mixture, wherein the swelling of the particles takes place in the
building material mixture itself.
Description
[0001] The present invention relates to the use of polymeric
microparticles in hydraulically setting building material mixtures
for the purpose of enhancing their frost resistance and cyclical
freeze/thaw durability.
[0002] Decisive factors affecting the resistance of concrete to
frost and to cyclical freeze/thaw under simultaneous exposure to
thawing agents are the imperviousness of its microstructure, a
certain strength of the matrix, and the presence of a certain pore
microstructure. The microstructure of a cement-bound concrete is
traversed by capillary pores (radius: 2 .mu.m-2 mm) and gel pores
(radius: 2-50 nm). Water present in these pores differs in its
state as a function of the pore diameter. Whereas water in the
capillary pores retains its usual properties, that in the gel pores
is classified as condensed water (mesopores: 50 nm) and
adsorptively bound surface water (micropores: 2 nm), the freezing
points of which may for example be well below -50.degree. C. [M. J.
Setzer, Interaction of water with hardened cement paste, Ceramic
Transactions 16 (1991) 415-39]. Consequently, even when the
concrete is cooled to low temperatures, some of the water in the
pores remains unfrozen (metastable water). For a given temperature,
however, the vapor pressure over ice is lower than that over water.
Since ice and metastable water are present alongside one another
simultaneously, a vapor-pressure gradient develops which leads to
diffusion of the still-liquid water to the ice and to the formation
of ice from said water, resulting in removal of water from the
smaller pores or accumulation of ice in the larger pores. This
redistribution of water as a result of cooling takes place in every
porous system and is critically dependent on the type of pore
distribution.
[0003] The artificial introduction of microfine air pores in the
concrete hence gives rise primarily to what are called expansion
spaces for expanding ice and ice-water. Within these pores,
freezing water can expand or internal pressure and stresses of ice
and ice-water can be absorbed without formation of microcracks and
hence without frost damage to the concrete. The fundamental way in
which such air-pore systems act has been described, in connection
with the mechanism of frost damage to concrete, in a large number
of reviews [Schulson, Erland M. (1998) Ice damage to concrete.
CRREL Special Report 98-6; S. Chatterji, Freezing of air-entrained
cement-based materials and specific actions of air-entraining
agents, Cement & Concrete Composites 25 (2003) 759-65; G. W.
Scherer, J. Chen & J. Valenza, Methods for protecting concrete
from freeze damage, U.S. Pat. No. 6,485,560 B1 (2002); M. Pigeon,
B. Zuber & J. Marchand, Freeze/thaw resistance, Advanced
Concrete Technology 2 (2003) 11/1-11/17; B. Erlin & B. Mather,
A new process by which cyclic freezing can damage concrete--the
Erlin/Mather effect, Cement & Concrete Research 35 (2005)
1407-11].
[0004] A precondition for improved resistance of the concrete on
exposure to the freezing and thawing cycle is that the distance of
each point in the hardened cement from the next artificial air pore
does not exceed a defined value. This distance is also referred to
as the "Powers spacing factor" [T. C. Powers, The air requirement
of frost-resistant concrete, Proceedings of the Highway Research
Board 29 (1949) 184-202]. Laboratory tests have shown that
exceeding the critical "Power spacing factor" of 500 .mu.m leads to
damage to the concrete in the freezing and thawing cycle. In order
to achieve this with a limited air-pore content, the diameter of
the artificially introduced air pores must therefore be less than
200-300 .mu.m [K. Snyder, K. Natesaiyer & K. Hover, The
stereological and statistical properties of entrained air voids in
concrete: A mathematical basis for air void systems
characterization, Materials Science of Concrete VI (2001)
129-214].
[0005] The formation of an artificial air-pore system depends
critically on the composition and the conformity of the aggregates,
the type and amount of the cement, the consistency of the concrete,
the mixer used, the mixing time, and the temperature, but also on
the nature and amount of the agent that forms the air pores, the
air entrainer. Although these influencing factors can be controlled
if account is taken of appropriate production rules, there may
nevertheless be a multiplicity of unwanted adverse effects,
resulting ultimately in the concrete's air content being above or
below the desired level and hence adversely affecting the strength
or the frost resistance of the concrete.
[0006] Artificial air pores of this kind cannot be metered
directly; instead, the air entrained by mixing is stabilized by the
addition of the aforementioned air entrainers [L. Du & K. J.
Folliard, Mechanism of air entrainment in concrete, Cement &
Concrete Research 35 (2005) 1463-71]. Conventional air entrainers
are mostly surfactant-like in structure and break up the air
introduced by mixing into small air bubbles having a diameter as
far as possible of less than 300 .mu.m, and stabilize them in the
wet concrete microstructure. A distinction is made here between two
types.
[0007] One type--for example sodium oleate, the sodium salt of
abietic acid or Vinsol resin, an extract from pine roots--reacts
with the calcium hydroxide of the pore solution in the cement paste
and is precipitated as insoluble calcium salt. These hydrophobic
salts reduce the surface tension of the water and collect at the
interface between cement particle, air and water. They stabilize
the microbubbles and are therefore encountered at the surfaces of
these air pores in the concrete as it hardens.
[0008] The other type--for example sodium lauryl sulfate (SDS) or
sodium dodecyl-phenylsulfonate--reacts with calcium hydroxide to
form calcium salts which, in contrast, are soluble, but which
exhibit an abnormal solution behavior. Below a certain critical
temperature the solubility of these surfactants is very low, while
above this temperature their solubility is very good. As a result
of preferential accumulation at the air/water boundary they
likewise reduce the surface tension, thus stabilize the
microbubbles, and are preferably encountered at the surfaces of
these air pores in the hardened concrete.
[0009] The use of these prior-art air entrainers is accompanied by
a host of problems [L. Du & K. J. Folliard, Mechanism of air
entrainment in concrete, Cement & Concrete Research 35 (2005)
1463-71]. For example, prolonged mixing times, different mixer
speeds and altered metering sequences in the case of ready-mix
concretes result in the expulsion of the stabilized air (in the air
pores).
[0010] The transporting of concretes with extended transport times,
poor temperature control and different pumping and conveying
equipment, and also the introduction of these concretes in
conjunction with altered subsequent processing, jerking and
temperature conditions, can produce a significant change in an
air-pore content set beforehand. In the worst case this may mean
that a concrete no longer complies with the required limiting
values of a certain exposure class and has therefore become
unusable [EN 206-1 (2000), Concrete--Part 1: Specification,
performance, production and conformity].
[0011] The amount of fine substances in the concrete (e.g. cement
with different alkali content, additions such as flyash, silica
dust or color additions) likewise adversely affects air
entrainment. There may also be interactions with flow improvers
that have a defoaming action and hence expel air pores, but may
also introduce them in an uncontrolled manner.
[0012] All of these influences which complicate the production of
frost-resistant concrete can be avoided if, instead of the required
air-pore system being generated by means of abovementioned air
entrainers with surfactant-like structure, the air content is
brought about by the admixing or solid metering of polymeric
microparticles (hollow microspheres) [H. Sommer, A new method of
making concrete resistant to frost and de-icing salts, Betonwerk
& Fertigteiltechnik 9 (1978) 476-84]. Since the microparticles
generally have particle sizes of less than 100 .mu.m, they can also
be distributed more finely and uniformly in the concrete
microstructure than can artificially introduced air pores.
Consequently, even small amounts are sufficient for sufficient
resistance of the concrete to the freezing and thawing cycle.
[0013] The use of polymeric microparticles of this kind for
improving the frost resistance and cyclical freeze/thaw durability
of concrete is already known from the prior art [cf. DE 2229094 A1,
U.S. Pat. No. 4,057,526 B1, U.S. Pat. No. 4,082,562 B1, DE 3026719
A1]. The microparticles described therein are notable in particular
for the fact that they possess a void which is smaller than 200
.mu.m (diameter), and this hollow core consists of air (or a
gaseous substance). This likewise includes porous microparticles on
the 100 .mu.m scale, which may possess a multiplicity of relatively
small voids and/or pores.
[0014] With the use of hollow microparticles for artificial air
entrainment in concrete, two factors proved to be disadvantageous
for the implementation of this technology on the market. On the one
hand the preparation costs of hollow microspheres in accordance
with the prior art are too high, and on the other relatively high
doses are required in order to achieve satisfactory resistance of
the concrete to freezing and thawing cycles.
[0015] The object on which the present invention is based was
therefore that of providing a means of improving the frost
resistance and cyclical freeze/thaw durability for hydraulically
setting building material mixtures that develops its full activity
even in relatively low doses. A further object was not, or not
substantially, to impair the mechanical strength of the building
material mixture as a result of said means.
[0016] These and also further objects, not identified explicitly
yet readily derivable or comprehensible from the circumstances
discussed herein in the introduction, are achieved by core/shell
microparticles which possess a base-swellable core and whose shell
is composed of polymers having a glass transition temperature of
below 50.degree. C.; preference is given to glass transition
temperatures of less than 30.degree. C.; particular preference is
given to glass transition temperatures of less than 15.degree. C.;
the most preference is given to glass transition temperatures of
less than 5.degree. C.
[0017] The particles of the invention are prepared preferably by
emulsion polymerization.
[0018] It has been found that the particles of the invention are
suitable for producing, even added in very small amounts, effective
resistance towards frost cycling and freeze/thaw cycling.
[0019] In one particularly preferred embodiment of the invention
the unswollen core/shell particles are added to the building
material mixture, and they swell in the strongly alkaline mixture
and so form the cavity `in situ`, as it were.
[0020] Also in accordance with the invention is a process for
preparing a building material mixture which involves mixing
swellable but as yet unswollen core/shell particles with the
typical components of a building material mixture and the swelling
of the particles taking place only in the building material
mixture.
[0021] According to one preferred embodiment the microparticles
used are composed of polymer particles which possess a core (A) and
at least one shell (B), the core/shell polymer particles having
been swollen by means of a base.
[0022] The preparation of these polymeric microparticles by
emulsion polymerization and their swelling using bases such as
alkali or alkali metal hydroxides and also ammonia and amine, for
example, are described in European patents EP 22 633 B1, EP 735 29
B1 and EP 188 325 B1.
[0023] The core (A) of the particle contains one or more
ethylenically unsaturated carboxylic acid (derivative) monomers
which permit swelling of the core; these monomers are preferably
selected from the group of acrylic acid, methacrylic acid, maleic
acid, maleic anhydride, fumaric acid, itaconic acid and crotonic
acid and mixtures thereof. Acrylic acid and methacrylic acid are
particularly preferred.
[0024] In one particular embodiment of the invention the polymers
that form the core may also be crosslinked. The amounts of
crosslinker employed with preference are 0-10% by weight (relative
to the total amount of monomers in the core); preference is further
given to 0-6% by weight of crosslinker; the most preferred are 0-3%
by weight. In any case, the amount of the crosslinker must be
selected such that swelling is not completely prevented.
[0025] Examples that may be mentioned of suitable crosslinkers
include ethylene glycol di(meth)acrylate, propylene glycol
di(meth)acrylate, allyl(meth)acrylate, divinylbenzene, diallyl
maleate, trimethylolpropane trimethacrylate, glycerol
di(meth)acrylate, glycerol tri(meth)acrylate, pentaerythritol
tetra(meth)acrylate or mixtures thereof.
[0026] The (meth)acrylate notation here denotes not only
methacrylate, such as methyl methacrylate, ethyl methacrylate,
etc., but also acrylate, such as methyl acrylate, ethyl acrylate,
etc., and also mixtures of both.
[0027] The shell (B) is composed predominantly of nonionic,
ethylenically unsaturated monomers. As monomers of this kind it is
preferred to use styrene, butadiene, vinyltoluene, ethylene, vinyl
acetate, vinyl chloride, vinylidene chloride, acrylonitrile,
acrylamide, methacrylamide and/or C1-C12 alkyl esters of
(meth)acrylic acid or mixtures thereof.
[0028] When selecting the monomers it is necessary in accordance
with the invention to ensure that the glass transition temperature
of the resulting copolymer is less than 50.degree. C.; preferably
the glass transition temperature is less than 30.degree. C.,
particular preference being given to glass transition temperatures
of less than 15.degree. C.; the most preferable are glass
transition temperatures of less than 5.degree. C.
[0029] The glass transition temperature is calculated in this case
appropriately with the aid of the Fox equation.
[0030] The Fox equation refers in this specification to the
following formula, which is known to the skilled worker:
1 Tg ( P ) = a Tg ( A ) + b Tg ( B ) + c Tg ( C ) +
##EQU00001##
[0031] In this formula Tg(P) designates the glass transition
temperature to be calculated for the copolymer, in degrees Kelvin.
Tg(A), Tg(B), Tg(C), etc. designate the respective glass transition
temperatures (in degrees Kelvin) of the high molecular mass
homopolymers of the monomers A, B, C, etc., measured by dynamic
heat-flow differential calorimetry (Dynamic Scanning Calorimetry,
DSC).
[0032] (Tg values for homopolymers are listed inter alia in, for
example, Polymer Handbook, Johannes Brandrup, Edmund H. Immergut,
Eric A. Grulke; John Wiley & Sons, New York (1999)).
[0033] The Fox equation has become established for the estimation
of the glass transition temperature, even though under certain
conditions there may be deviations from values measured.
[0034] For a more precise determination of the glass transition
temperature it is possible to prepare the shell polymer separately;
the glass transition temperature can then be measured with the aid
of DSC (read off from the second heating curve, heating or cooling
raterate 10 K/min).
[0035] In addition to the abovementioned monomers it is possible
for the polymer envelope (B) to contain monomers, which enhances
the permeability of the shell for bases--and here, especially,
ionic bases. These may be, on the one hand, acid-containing
monomers such as acrylic acid, methacrylic acid, maleic acid,
maleic anhydride, fumaric acid, monoesters of fumaric acid,
itaconic acid, crotonic acid, maleic acid, monoesters of maleic
acid, acrylamidoglycolic acid, methacrylamidobenzoic acid, cinnamic
acid, vinylacetic acid, trichloroacrylic acid,
10-hydroxy-2-decenoic acid, 4-methacryloyloxyethyltrimethylic acid,
styrenecarboxylic acid, 2-(isopropenylcarbonyloxy)ethanesulfonic
acid, 2-(vinylcarbonyloxy)ethanesulfonic acid,
2-(isopropenylcarbonyloxy)propylsulfonic acid,
2-(vinylcarbonyloxy)propylsulfonic acid,
2-acrylamido-2-methylpropanesulfonic acid,
acrylamidododecanesulfonic acid, 2-propene-1-sulfonic acid,
methallylsulfonic acid, styrenesulfonic acid, styrenedisulfonic
acid, methacrylamidoethanephosphonic acid, vinylphosphonic acid,
and mixtures thereof. On the other hand it is also possible for the
permeability to be enhanced by means of hydrophilic, nonionic
monomers, of which mention should be made here, as examples, of
acrylonitrile, (meth)acrylamide, cyano-methyl methacrylate,
N-vinylamides, N-vinylformamides, N-vinylacetamides,
N-vinyl-N-methylacetamides, N-vinyl-N-methylformamides,
N-methylol(meth)acrylamide, vinylpyrrolidone,
N,N-dimethylpropylacrylamide, dimethyl-acrylamide, and also other
hydroxyl-, amino-, amido- and/or cyano-containing monomers, and
mixtures thereof.
[0036] A restriction of these or other monomers not specified at
this point exists only by virtue of the fact that the glass
transition temperatures according to the invention are not exceeded
and the monomer mixture ought not to stand in the way of the
preparation and the ordered construction of the article.
[0037] Hydrophilic and acid-containing monomers together typically
account for not more than 30% by weight (relative to the total
monomer mixture of the shell) of the composition of the polymer
envelope (B); particular preference is given to amounts between
0.2% and 20% by weight, the most preference to amounts between 0.5%
and 10% by weight.
[0038] In a further preferred embodiment the monomer composition of
the core and of the shell does not change with a sharp
discontinuity, as is the case for a core/shell particle of ideal
construction, but instead changes gradually in two or more steps or
in the form of a gradient.
[0039] Where the microparticles are constructed as multishell
particles, the composition of the shells lying between core and
outer shell is often oriented on the shells adjacent to either
side, which means that the amount of a monomer Mx in general
between the amount M(x+1) in the next-outer shell (which may also
be the outer shell) and the amount M(x-1) in the next-inner shell
(or the core). This is not mandatory, however, and in further
particular embodiments the compositions of such intermediate shells
may also be selected freely, provided it does not stand in the way
of the preparation and the ordered construction of the
particle.
[0040] The shell B of the particles of the invention accounts for
preferably 10% to 96% by weight of the total weight of the
particle, particular preference being given to shell fractions of
20% to 94% by weight. The most preferred are shell fractions of 30%
to 92% by weight.
[0041] In the case of very thin shells this may lead to the shells
of the particles bursting on swelling. It has been found, however,
that this does not automatically result in the effect of these
particles being lost. In particular embodiments of the invention,
and especially when swelling takes place in the building material
mixture, this effect may be advantageous, since without the
restriction of the shell it is possible for better swelling of the
particles to take place.
[0042] Where the microparticles are swollen only in the building
mixture itself, it is possible to prepare dispersions having
significantly higher solids contents (i.e. weight fractions of
polymer relative to total weight of the dispersion), since the
volume occupied by the unswollen particles is of course smaller
than that of the swollen particles.
[0043] The polymer particles can also be initially swollen with a
small amount of base, and can be added in this partly swollen state
to the building material mixture. This corresponds, then, to a
compromise, since a somewhat lower raising of the solids content is
still always possible, while on the other hand the time which is
provided for swelling in the building material mixture can be made
shorter.
[0044] The polymer content of the microparticles used may be,
depending on diameter and on water content, 2% to 98% by weight
(weight of polymer relative to the total weight of the water-filled
particle).
[0045] Preference is given to polymer contents of 5% to 60% by
weight, particular preference to polymer contents of 10% to 40% by
weight.
[0046] The microparticles of the invention can be prepared
preferably by emulsion polymerization and preferably have an
average particle size of 100 to 5000 nm; particular preference is
given to an average particle size of 200 to 2000 nm. The most
preferred are average particle sizes of 250 to 1000 nm.
[0047] The average particle size is determined by means for example
of counting a statistically significant amount of particles, using
transmission electron micrographs.
[0048] In the case of preparation by emulsion polymerization the
microparticles are obtained in the form of an aqueous dispersion.
Accordingly, the addition of the microparticles to the building
material mixture takes place preferably likewise in this form.
[0049] Within the scope of the present invention it is also readily
possible, however, to add the water-filled microparticles directly
as a solid to the building material mixture. For that purpose the
microparticles are, for example, coagulated and isolated from the
aqueous dispersion by standard methods (e.g. filtration,
centrifugation, sedimentation and decanting) and the particles are
subsequently dried.
[0050] If addition in solid form is desired or necessary for
technical reasons associated with processing, then further
preferred methods of drying are spray drying and freeze drying.
[0051] The water-filled microparticles are added to the building
material mixture in a preferred amount of 0.01% to 5% by volume, in
particular 0.1% to 0.5% by volume. The building material mixture,
in the form for example of concrete or mortar, may in this case
include the customary hydraulically setting binders, such as
cement, lime, gypsum or anhydrite, for example.
[0052] A substantial advantage through the use of the water-filled
microparticles is that only an extremely small amount of air is
introduced into the concrete. As a result, significantly improved
compressive strengths are achievable in the concrete. These are
about 25%-50% above the compressive strengths of concrete obtained
with conventional air entrainment. Hence it is possible to attain
strength classes which can otherwise be set only by means of a
substantially lower water/cement value (w/c value). Low w/c values,
however, in turn significantly restrict the processing properties
of the concrete in certain circumstances.
[0053] Moreover, higher compressive strengths may result in it
being possible to reduce the cement content of the concrete that is
needed for strength to develop, and hence a significant reduction
in the price per m.sup.3 of concrete.
* * * * *